16 HEREDITY IN RELATION TO EUGENICS 



3. The Laws of Heredity 



We are now in a position to understand the modern laws 

 of heredity. First of all it will be recognized that nothing 

 is inherited except the determiners in the germ cells; the 

 characters themselves, on the contrary, are not directly 

 inherited. A clear grasp of this fact gives the answer to 

 many questions. Thus the possibility of the transmission 

 of somatic mutilations is seen to depend upon the capacity 

 of such mutilations to modify the determiners in the germ 

 plasm, and such capacity has never been proved. On the 

 other hand, the germ cells receive nutritive and other par- 

 ticles from the blood and they may receive also poisons 

 from it. Hence arises the possibility of depauperization 

 of the germ plasm and of ''race poisons;" but these are 

 exceptional and little known phenomena. 



To understand the way heredity acts, let us take the case 

 where both germ cells that unite to produce the fertihzed 

 egg carry the determiner for a unit character, A. Then 

 in the child that develops out of that fertilized egg there 

 is a double stimulus to the development of the unit char- 

 acter A. We say the character is of duplex origin. If, on 

 the other hand, only one germ cell, say the egg, has the 

 determiner of a character while the other, the sperm, lacks 

 it, then in the fertilized egg the determiner is simplex and 

 the resulting character is of simplex origin. Such a char- 

 acter is often less perfectly developed than the corresponding 

 character of duplex origin (Fig. 6). Finally, if neither 

 germ cell carries the determiner of the character A, it will 

 be absent in the embryo and the developed child. A per- 

 son who shows a character in his body (soma) may or may 

 not have the determiner for that character in all of the ripe 

 germ cells he carries, but a person who lacks a given unit 

 character ordinarily lacks the corresponding determiner 



